INFRARED SPECTROSCOPY OF OH AND OH-C$_2$H$_2$ EMBEDDED IN HELIUM NANODROPLETS

Abstract

The hydroxyl radical and its complex with acetylene have been characterized in superfluid helium nanodroplets by infrared depletion spectroscopy. For OH, two sharp Q(3/2) lines were observed, with a separation that is consistent with a fivefold increase in the parity (lambda type) doubling of the $^{2}\Pi_{3/2}$ state in helium droplets relative to the gas phase. This increase is rationalized in terms of the differences in the potential energy surfaces for He-OH(A$^\prime$) and He-OH(A$^{\prime\prime}$), which are most pronounced at around 90$^irc$. Switching to the T-shaped OH-C$_2$H$_2$ complex, depletion signals corresponding to OH and CH stretching transitions were observed. The spectra reveal that the electronic angular momentum of OH in the complex is only partially quenched, and to a similar degree as observed in the gas phase, \textbf{121}, 3009 (2004).}$^{,}$, \textbf{121}, 5845 (2004).}. This indicates that the helium droplet environment does not significantly affect the electronic intermolecular interactions in OH-C$_2$H$_2$.